JP5164147B2 - Scanning probe microscope and cantilever drive device - Google Patents

Description

  The present invention relates to a scanning probe microscope including a cantilever and a cantilever driving device.

  Conventionally, as a typical scanning probe microscope (SPM), a scanning tunneling microscope (STM) and an atomic force microscope (AFM) are known. Among these, the AFM includes a cantilever having a probe at a free end, a sensor for detecting the displacement of the cantilever, and a sample stage scanner. The sensor is typically an optical lever type sensor. The sample stage scanner is generally composed of a piezo element, and moves the sample in the X, Y, and Z directions with respect to the cantilever.

  In the AFM, the sample and the cantilever are relatively scanned in the XY directions. At this time, the displacement in the Z direction is feedback controlled so that the force applied to the cantilever and the sample is kept constant. This feedback control of the displacement in the Z direction is called Z scanning. The fine shape of the sample surface can be obtained from the movement of the sample stage to keep the force applied to the cantilever and the sample constant.

  As an AFM measurement mode, an AC mode and a contact mode are typically known. In the AC mode, the cantilever is excited at the resonance frequency. As the cantilever approaches the sample, the amplitude changes. Therefore, feedback control in the Z direction is performed so that the amplitude is constant, and thereby the distance between the cantilever and the sample is kept constant.

“AFM scanning speed”
In the conventional AFM, the measurement is slow and it takes a time of the order of minutes to take one image. This is because feedback scanning is slow. The feedback operation is to keep the force applied to the cantilever probe and the sample constant by moving the sample stage up and down to adjust the distance between the probe and the sample surface. Since it takes time to acquire an image, it is difficult to observe the movement of the sample, for example.

  The feedback scanning loop involves most AFM devices. Related devices are cantilevers, sensors, sensor amplifiers, control circuits, piezo drive power supplies, sample stage scanners, and the like. Of these devices, the sample stage scanner is usually the slowest device. Therefore, in order to increase the shooting speed of AFM, it is essential to increase the speed of the scanner.

  However, in the conventional AFM, the limit of speeding up the scanner is low. More specifically, in the conventional AFM, the scanner is constituted by a piezo element (piezoelectric element) having a macro size. The sample stage is scanned in a three-dimensional direction of XYZ by a scanner. The feedback scan is a Z-direction scan, and is a faster scan than the XY-direction scan. This Z-direction scanning needs to be performed in a region below the resonance frequency of the piezo element. Therefore, in order to increase the scanning speed, it is necessary to increase the resonance frequency of the Z-direction scanning piezo element. However, since the piezo element has a macro size compared to a cantilever or the like, the resonance frequency of the piezo element is low. This is a factor that hinders the speeding up of the scanner.

  In feedback scanning, the cantilever may be moved in the Z direction instead of moving the sample stage in the Z direction. Therefore, a cantilever having a self-actuation function has been developed by introducing a piezoelectric thin film into the cantilever using MEMS technology. In this case, since the cantilever is much smaller than the sample stage scanner, the resonance frequency can be easily increased. Therefore, it is easy to speed up the feedback scanning. However, since the self-actuation function is provided, the structure of the cantilever becomes complicated, and the cantilever becomes very hard. For this reason, the self-actuation type cantilever has a limitation that it is difficult to use for measuring fragile and soft samples such as biopolymers and synthetic polymers.

“Cantilever Q-factor control”
By the way, the cantilever is a kind of resonance system. As a quantity representing the sharpness of the resonance spectrum (the relationship of the displacement of the cantilever with the frequency of the excitation force), there is a Q factor (Quality factor). The Q value increases as the viscous resistance applied to the resonance system decreases. Conversely, when the viscous resistance is large, the Q value becomes small.

  The response speed of the resonance system is represented by πf / Q. Here, f is a resonance frequency. The larger the Q value, the slower the response speed. On the other hand, regarding the displacement sensitivity of the resonance system with respect to external force, the displacement sensitivity increases as the Q value increases.

Q value control for artificially changing the Q value of the cantilever has already been proposed. The principle of Q value control is as follows. The equation of motion of the resonant system is shown below.

The displacement x is detected, the displacement x is time differentiated, and the time differentiation is multiplied by a coefficient α. This value is added to or subtracted from the excitation force F (t) to obtain the following expression.

  Here, in the case of addition (+), the viscous resistance decreases, and as a result, the Q value increases. In the case of subtraction (-), the viscous resistance increases and the Q value decreases. In this way, the Q value can be arbitrarily increased or decreased. Thereby, the response speed and the displacement sensitivity can be adjusted.

  The Q value control of the cantilever is control for applying an external force to the cantilever, as shown in the above formula, and it is conceivable to apply an external force to the cantilever via some medium. However, since a phase delay occurs, the phase does not accurately become 90 degrees even though the phase should change 90 degrees by time differentiation. Therefore, control is not easy. When an external force is applied through the medium, surrounding mechanical elements also have a resonance frequency. Therefore, it becomes difficult to accurately match the frequency of the external force with the resonance frequency of the cantilever.

  Considering such matters, it is desirable to apply an external force directly to the cantilever in the Q value control. Therefore, conventionally, a ferromagnetic material is attached to the cantilever. Alternatively, the cantilever is coated with a ferromagnetic thin film. An external force is applied to the cantilever by the electromagnet. Even in a cantilever having a self-actuation function, an external force can be directly applied to the cantilever. However, in such a configuration, special processing of the cantilever is necessary, and the mechanical characteristics (resonance frequency and spring constant) of the cantilever change.

  A related technique is disclosed in Japanese Patent Application Laid-Open No. 2004-212078. In this related technique, the cantilever is irradiated with laser light, and the cantilever is excited by the laser light.

  As described above, in the conventional AFM, the scanning speed in the Z direction is slow, and therefore the measurement speed is slow. Further, the conventional AFM has room for improvement in the Q value control.

  The present invention has been made under the above background. One of the objects of the present invention is to provide a scanning probe microscope capable of improving the scanning speed.

  Another object of the present invention is to provide a scanning probe microscope capable of suitably controlling the Q value.

  The scanning probe microscope of the present invention includes a cantilever, a sensor for detecting the displacement of the cantilever, a light irradiation unit that irradiates light to the cantilever to cause thermal expansion deformation, and controls the light intensity of the light irradiation unit. A light irradiation control unit that changes the displacement of the cantilever, and the light irradiation control unit performs feedback control of the cantilever by controlling the light intensity of the light irradiation unit based on the displacement detected by the sensor.

  With this configuration, the cantilever is irradiated with light, and the cantilever is displaced by thermal expansion deformation. Scanning in the Z direction is performed using this displacement. The cantilever is very small and has a high resonance frequency. Therefore, the resonance frequency of the Z scanning configuration is increased, and the scanning speed can be increased. Moreover, it can avoid that a cantilever becomes hard like the conventional self-actuation type cantilever.

  The light irradiation control unit may include a thermal response compensation unit that compensates for a delay in the thermal response of the cantilever with respect to the light irradiation from the light irradiation unit. The thermal response compensator may have a configuration equivalent to the inverse transfer function of the cantilever heat transfer function (meaning a function that is the inverse of the heat transfer function, the same applies hereinafter). Thermal response compensation can apparently eliminate (or significantly reduce) the slow process of heat conduction. The element that connects light irradiation as input and displacement as output becomes close to the original mechanical characteristics of the cantilever. Since the response speed of deformation due to thermal expansion is increased, high-speed scanning and high-speed imaging are possible.

  Further, the light irradiation control unit may further vibrate the cantilever by controlling the light intensity of the light irradiation unit. The intensity of light may be controlled so as to fluctuate with a vibration waveform (typically a vibration waveform according to the resonance frequency of the cantilever) corresponding to the vibration to be generated by the cantilever. By light irradiation, both excitation of the cantilever and Z scanning are performed. Therefore, the structure can be simplified.

  Further, the light irradiation control unit may further include a Q value control unit that performs Q value control based on the displacement detected by the sensor. The Q value control unit may have a configuration for obtaining a value obtained by adding a gain to the differential of the displacement detected by the sensor (the gain corresponds to a coefficient multiplied by the differential of the displacement by the Q value control). With this configuration, the Q value can be controlled using light irradiation.

  In another aspect of the present invention, the scanning probe microscope includes a cantilever, a light irradiation unit that irradiates light to the cantilever to cause thermal expansion deformation, and controls the light intensity of the light irradiation unit to control the displacement of the cantilever. A light irradiation control unit that changes the oscillation control unit that vibrates the cantilever by controlling the light intensity of the light irradiation unit, and the thermal response of the cantilever to the light irradiation from the light irradiation unit. And a thermal response compensator for compensating for the delay.

  In this embodiment, light irradiation is used to excite the cantilever. This aspect includes a configuration in which light irradiation is applied to Z scanning and a configuration in which light irradiation is not applied. Also in this aspect, the heat conduction process can be apparently eliminated (or significantly shortened) by the thermal response compensation, the mechanical characteristics of the cantilever remain, and the excitation can be suitably controlled. Also in this aspect, the Q value control may be suitably performed based on the displacement of the cantilever. Furthermore, various other features of the present invention are also applicable to this embodiment. This aspect also provides the advantages of the present invention.

  Another aspect of the present invention is an observation method using a scanning probe microscope. In this method, the cantilever and the sample are moved relative to each other in the X and Y directions, light is applied to the cantilever to cause thermal expansion deformation, the displacement of the cantilever is detected, and the intensity of the light is detected based on the detected displacement of the cantilever. The cantilever feedback control is performed by controlling. The light intensity control may compensate for a delay in the thermal response of the cantilever with respect to light irradiation. Light intensity control may be utilized for cantilever vibration. The light intensity control may be Q value control based on the displacement of the cantilever. Furthermore, various other features of the present invention are also applicable to this embodiment. This aspect also provides the advantages of the present invention.

  Another aspect of the present invention is an observation method using a scanning probe microscope. In this method, the cantilever and the sample are moved relative to each other in the X and Y directions, light is irradiated to the cantilever to cause thermal expansion deformation, the cantilever is vibrated by controlling the light intensity, and the light is controlled by controlling the light intensity. Compensate for delay in thermal response of cantilever to irradiation. The light intensity control may be Q value control based on the displacement of the cantilever. Furthermore, various other features of the present invention are also applicable to this embodiment. This aspect also provides the advantages of the present invention.

  Within the scope of the present invention, the expression of the present invention need not be limited to the scanning probe microscope described above and the observation method using the same. Another aspect of the present invention may be, for example, an atomic force microscope and an observation method using the same. Another aspect of the present invention may be a driving device or a driving method of a scanning probe microscope or an atomic force microscope. Another aspect of the present invention may be a cantilever driving device or a cantilever driving method. Furthermore, another aspect of the present invention may be a control device or a control method for the above-described device. These aspects may include a configuration for driving the cantilever described above. Furthermore, various other features of the present invention are also applicable to these embodiments. These aspects also provide the advantages of the present invention.

  According to the above-described present invention, it is possible to improve the scanning speed by making good use of light irradiation, and it is possible to facilitate the Q value control.

  As described below, there are other aspects of the present invention. Accordingly, this disclosure is intended to provide some aspects of the invention and is not intended to limit the scope of the invention described and claimed herein.

FIG. 1 is a diagram showing a configuration of an AFM according to the present embodiment. FIG. 2 is a diagram showing deformation of the cantilever by light irradiation. FIG. 3 is a diagram illustrating an example of a cantilever. FIG. 4 is a diagram illustrating a process from light irradiation to lever displacement. FIG. 5 is a diagram showing the thermal response characteristics of the cantilever. FIG. 6 is a diagram illustrating a configuration of the thermal response compensation circuit. FIG. 7 is a diagram illustrating the effect of thermal response compensation. FIG. 8 is a diagram illustrating the effect of thermal response compensation. FIG. 9 is a diagram illustrating the effect of thermal response compensation. FIG. 10 is a diagram illustrating the effect of thermal response compensation. FIG. 11 is a diagram illustrating the effect of thermal response compensation. FIG. 12 is a diagram illustrating a configuration of an AFM according to another embodiment. FIG. 13 is a diagram illustrating a configuration of an AFM according to another embodiment. FIG. 14 is a diagram showing the configuration of the Q value control circuit. FIG. 15 is a diagram illustrating the effect of Q value control. FIG. 16 is a diagram illustrating the effect of Q value control. FIG. 17 is a diagram illustrating the effect of Q value control. FIG. 18 is a diagram illustrating the effect of Q value control.

  The detailed description of the present invention will be described below. However, the following detailed description and the accompanying drawings do not limit the invention. Instead, the scope of the invention is defined by the appended claims.

  In the present embodiment, the present invention is applied to an atomic force microscope (AFM).

  FIG. 1 shows the configuration of the AFM according to the present embodiment. The AFM 1 has, as a whole structure, a sample stage 3 that holds a sample, a cantilever 5, an XY scanner 7 that drives the sample stage 3 in the XY direction, a sensor 9 that detects displacement of the cantilever 5, and a cantilever 5 A drive laser unit 11 that performs light irradiation, a drive laser control unit 13 that controls the drive laser unit 11, and a computer 15 that controls the entire apparatus.

  The cantilever 5 is made of silicon nitride and has a probe at the free end. The light irradiation surface (the lower side of the figure) of the cantilever 5 is coated with gold. The light irradiation surface is irradiated with a sensor laser beam for detecting displacement and a driving laser beam for driving the lever.

  The cantilever 5 is held by a holder 21, and the holder 21 is excited by an oscillation circuit 23. In response to the excitation signal supplied from the oscillation circuit 23, the vibrator of the holder 21 vibrates, and thereby the cantilever 5 vibrates. The oscillation circuit 23 is controlled by the computer 15 so that the cantilever 5 vibrates in the vicinity of the resonance frequency.

  The XY scanner 7 is composed of a piezo element (piezoelectric element). The XY scanner 7 is controlled by an XY scan control circuit 25. An XY scanning (raster scanning) control signal is input from the computer 15 to the XY scanning control circuit 25. The XY scanning control circuit 25 drives the XY scanner 7 according to the control signal. Thereby, the sample stage 3 moves in the XY directions, and scanning in the XY directions is realized.

  The sensor 9 and the sensor laser unit 27 constitute an optical lever type displacement sensor. The sensor laser unit 27 irradiates the cantilever 5 with sensor laser light. The sensor laser light is red laser light (wavelength 670 nm). The sensor laser light is reflected by the gold coating surface of the cantilever 5 and reaches the sensor 9. The sensor 9 is composed of a photodiode and outputs a signal representing the displacement of the cantilever 5. In the figure, the configuration of an optical system such as a lens related to the sensor is omitted.

  The drive laser unit 11 irradiates the cantilever 5 with drive laser light. The driving laser light is a visible light laser in this embodiment. The wavelength of the drive laser light is set to a value that increases the light absorption capability of the gold coat. Specifically, the driving laser beam is a violet laser beam and the wavelength is 405 nm. When the drive laser light is irradiated, the cantilever 5 is thermally expanded. Since the thermal expansion coefficient is different between the silicon nitride and the gold coat constituting the cantilever 5, the cantilever 5 functions as a bimetal. Then, as shown in FIG. 2, the cantilever 5 is bent and deformed. Thereby, the displacement of the cantilever 5 in the Z direction is obtained. In the figure, the configuration of an optical system such as a lens related to the drive laser is omitted.

  The drive laser unit 11 is controlled by a drive laser control unit 13. The drive laser control unit 13 controls the drive laser unit 11 by supplying an irradiation control signal to the drive laser unit 11. The irradiation control signal is a modulation input signal. Intensity modulation is applied to the laser in accordance with the modulation input signal. The modulation intensity is used for controlling the displacement of the cantilever 5. The amount of deformation of the cantilever 5 changes according to the laser light intensity, and the displacement in the Z direction changes.

  In the present embodiment, the drive laser control unit 13 controls the displacement of the cantilever 5 by controlling the modulation intensity of the drive laser beam, thereby functioning as a Z scan control unit as described below.

  The drive laser controller 13 receives signals of a reference displacement and a reference amplitude from the computer 15. The reference displacement is an initial displacement of the cantilever 5 and can also be referred to as an offset displacement. The reference amplitude is the amplitude of the cantilever 5 that is a target of feedback control. Further, the displacement of the cantilever 5 is input from the sensor 9 to the drive laser controller 13. The drive laser controller 13 controls the laser beam intensity according to the reference displacement, and performs feedback control of the laser beam intensity so that the lever amplitude matches the reference amplitude based on the reference amplitude and the lever displacement. Thereby, the amplitude of the cantilever 5 is kept constant.

  The drive laser control unit 13 includes an amplitude detection circuit 31 and a feedback circuit 33 for feedback control. The amplitude detection circuit 31 obtains the amplitude of the cantilever 5 by processing the displacement signal of the cantilever 5 input from the sensor 9. Then, the amplitude detection circuit 31 outputs a detected amplitude signal to the feedback circuit 33.

  The feedback circuit 33 includes a subtracter that generates a deviation signal by subtracting the detected amplitude signal from the reference amplitude signal, and a PID circuit that amplifies the deviation signal. With these configurations, a feedback signal is generated.

  The feedback signal is added to the reference displacement signal input from the computer 15. The irradiation control signal thus obtained is supplied to the thermal response compensation circuit 35. The thermal response compensation circuit 35 is a circuit that compensates for a delay in thermal response of the cantilever 5 with respect to light irradiation. The thermal response compensation circuit 35 will be described later.

  The irradiation control signal that has passed through the thermal response compensation circuit 35 is supplied to the drive laser unit 11. Then, the intensity of the drive laser light varies according to the irradiation control signal, and the displacement of the cantilever 5 varies. Since the irradiation control signal is made from the feedback control signal obtained by processing the amplitude signal obtained based on the lever displacement, the laser beam intensity varies according to the feedback signal, and the displacement of the cantilever 5 in the Z direction is also fed back. It varies depending on the signal. In this way, feedback control of the cantilever 5 is realized.

  The computer 15 controls the entire AFM 1. The computer 15 also provides a user interface function. Various user instructions are input to the computer 15, and the computer 15 controls the AFM 1 in accordance with the user input. Further, the computer 15 receives a feedback signal from the feedback circuit 33 of the drive laser control unit 13. The computer 15 generates a three-dimensional image of the sample surface based on the XY scanning control data and the input feedback signal, and displays the three-dimensional image on the monitor 41.

  The configuration of each part of the AFM 1 has been described above. The above configuration may be modified as long as the same function can be obtained. For example, the drive laser control unit 13 may be built in the computer 15. Software may be used to realize the function of the drive laser controller 13. Data processed in the computer 15 may correspond to the above-described signal.

  Next, the overall operation of the AFM 1 will be described. The computer 15 sends an XY scanning control signal to the XY scanning control circuit 25 to drive the sample stage 3 in the XY directions. Further, the computer 15 generates an excitation signal in the oscillation circuit 23 and vibrates the cantilever 5 together with the holder 21. Furthermore, the computer 15 supplies the drive laser controller 13 with signals of reference displacement and reference amplitude. The drive laser controller 13 functions as described above and performs feedback control of the cantilever 5 so as to deform the cantilever 5 in the Z direction to the reference displacement.

  In this way, with the cantilever 5 vibrating, the sample is scanned in the XY direction, and the cantilever 5 is scanned in the Z direction so as to keep the amplitude constant. A feedback signal from the drive laser controller 13 is supplied to the computer 15. The computer 15 generates a three-dimensional image of the sample surface based on the XY scanning control data and the input feedback signal. The generated three-dimensional image is displayed on the monitor 41.

  FIG. 3 shows the cantilever 5 in a specific example of the present embodiment, which is made from an electron micrograph (15000 times) of the cantilever. In this example, the cantilever 5 has a substantially uniform cross section as a whole, and the cross-sectional shape in the width direction is a rectangle. The tip of the lever has a tapered shape. The length of the lever is about 8 μm, the width is about 2 μm, and the thickness (including the gold coat) is about 0.1 μm. The gold coat is applied on the opposite side of the probe.

  In this example, the resonance frequency of the cantilever is about 800 kHz in the liquid and about 2.2 MHz in the atmosphere, and the resonance frequency is very high. Z scanning of the cantilever 5 is possible in a region lower than the resonance frequency. Therefore, the scanning speed can be greatly increased as compared with a conventional piezo element scanner (about 1 kHz). As can be seen from this example, the piezoelectric element is large, so that the resonance frequency is low and it is difficult to drive at high speed, whereas the cantilever is small, so the resonance frequency is high and high speed driving is easy.

  In the specific example of the present embodiment, the wavelength of the drive laser light is 405 nm. The laser can be modulated in intensity from 0 to 20 mW and can be used at a maximum of 50 MHz. In this case, the sensitivity was 5-8 nmpp / mW. The maximum movable displacement was 100 to 160 nmpp (when the modulation intensity was 20 mW). These values were obtained in a frequency band lower than the resonance frequency. Therefore, sufficient sensitivity and displacement can be obtained as an AFM.

  In the present embodiment, an actuation function can be easily obtained by irradiating a normal cantilever with light. That is, the complicated processing found in conventional self-actuation type cantilevers is unnecessary. Further, the increase in hardness seen in the conventional self-actuation type can also be avoided (in the cantilever of this embodiment, the spring constant is about 150 pN / nm). Furthermore, in the self-actuation type cantilever, securing insulation is also a problem. However, according to the present embodiment, the problem of insulation can be easily avoided.

"Thermal response compensation"
Next, the thermal response compensation circuit 35 will be described in detail. FIG. 4 shows a process from the input of light irradiation to the output of cantilever displacement. The upper process of FIG. 4 is a process when thermal response compensation is not performed. Light is absorbed by the cantilever, heat conduction occurs, a thermal expansion force is generated, the cantilever is deformed, and displacement occurs. In this series of processes, light absorption is a fast process (nanosecond or less). However, heat conduction is a slow process. Therefore, the response of lever displacement to light irradiation is delayed due to heat conduction. This delay in response is disadvantageous for speeding up Z scanning. Therefore, in this embodiment, thermal response compensation is performed. More specifically, the principle of phase compensation is introduced to realize thermal response compensation. As a result, as shown in the lower part of FIG. 4, the process of heat conduction apparently disappears and the response speed is increased.

  FIG. 5 shows the relationship between time and displacement when the cantilever is irradiated with laser light (thermal response compensation is not performed). The example of FIG. 5 shows a measurement result when the cantilever of the above-described specific example is used. As shown in FIG. 5, the displacement of the cantilever gradually increases with time, and the thermal response is slow. This delay is due to heat conduction.

  Here, the thermal response of FIG. 5 behaves like a low-pass filter of an electronic circuit. That is, the thermal response delay phenomenon is similar to the delay of the low-pass filter. In such a case, phase compensation is preferably applied in the electronic circuit. This embodiment pays attention to this point, and introduces the principle of phase compensation as found in an electronic circuit into the heat conduction process, as will be described below. This new technique realizes thermal response compensation for cantilevers.

伝達関数Ｇ（ｓ）は、さらに、以下のように変形される。

In the example of FIG. 5, it can be considered that the heat conduction process consists of two processes. The heat transfer process can be regarded as two parallel low-pass filters. In this case, the transfer function G (s) is represented by the sum of two low-pass filters as shown in the following equation.

The transfer function G (s) is further modified as follows.

  The equation after the above transformation is the product of two low-pass filters and a (1 + derivative) term. This means that the transfer function G (s) is equivalent to two low-pass filters and one (1 + derivative) term in series. In phase compensation, an inverse transfer function of the transfer function G (s) is used. The inverse transfer function is the product of two types of (1 + derivative) terms and one low-pass filter. The thermal response compensation circuit 35 in FIG. 1 is configured by a circuit equivalent to the above inverse transfer function.

  FIG. 6 shows the configuration of the thermal response compensation circuit 35. As illustrated, the thermal response compensation circuit 35 includes one low-pass filter 35a and two (1+ differentiation) circuits 35b and 35c, which are arranged in series. The parameters of the circuits 35a, 35b, and 35c are set based on the measurement result of the thermal response of the cantilever 5 (FIG. 5). More specifically, a heat transfer function equivalent to the actual thermal response is obtained from the measurement result, and its inverse transfer function (a function that is the inverse of the heat transfer function) is obtained. A circuit is configured to correspond to the inverse transfer function.

  A drive laser light irradiation control signal (a signal obtained by synthesizing a feedback signal) is input to the thermal response compensation circuit 35 and then supplied to the drive laser unit 11 as shown in FIG. Thereby, the phase compensation shown by the above-mentioned principle is suitably realized.

  FIG. 7 shows the effect of phase compensation. FIG. 7 shows changes in lever displacement when laser light is irradiated, as in FIG. As a result of the phase compensation, the displacement of the lever reaches a constant value in a short time, and the response speed is increased.

  FIG. 8 is another diagram showing the effect of phase compensation. FIG. 8 shows the vibration characteristics of the cantilever in water. The horizontal axis is frequency and the vertical axis is amplitude. A line L1 (dotted line) in the figure is a characteristic of the model in calculation. The line L2 is a characteristic when piezo driving is performed. Line L3 is a characteristic when the lever is driven by laser light without phase compensation. Line L4 is a characteristic of laser driving when phase compensation is performed.

  As shown in the figure, by applying the phase compensation, the characteristics of the cantilever almost coincide with the calculated model. This means that the element that connects light irradiation as input and displacement as output is only the mechanical characteristics of the cantilever itself. Therefore, the response speed of lever displacement with respect to light irradiation is increased.

  Further, as shown in FIG. 8, in the present embodiment, a cantilever in water is driven by laser light, and the resonance frequency is about 800 kHz, which is very high. In a region below the resonance frequency, Z scanning can be performed. This means that the scanning speed of the Z scanning can be greatly increased as compared with the prior art.

  9 to 11 are still other diagrams showing the effect of phase compensation. 9 to 11 show the rectangular wave response of the cantilever displacement. FIG. 9 shows measurement data, and FIGS. 10 and 11 show simulation results. The upper stage in the figure is the input to the thermal response compensation circuit 35, the middle stage is the output of the circuit 35, and the lower stage is the displacement of the cantilever. In FIG. 9 and FIG. 10, step input is performed to the thermal response compensation circuit 35 at 10 μs, and the laser intensity is changed like a rectangular wave. In FIG. 11, similarly, the laser intensity is changed at 20 μs. Also in these figures, an increase in response speed due to phase compensation appears.

  The preferred embodiments of the present invention have been described above. In the present embodiment, the drive laser control unit 13 is a form of the light irradiation control unit, and in particular, the feedback circuit 33 functions as a Z scanning control unit. The thermal response compensation circuit 35 is one form of a thermal response compensation unit.

  According to the present embodiment, the cantilever is irradiated with light, and the cantilever is displaced by thermal expansion deformation. Scanning in the Z direction is performed using this displacement. Accordingly, it is possible to increase the scanning frequency by increasing the operating frequency of the Z scanning configuration. Further, it is possible to avoid hardening the cantilever as seen in the conventional self-actuation type cantilever.

  The cantilever has a plurality of layers having different coefficients of thermal expansion. Depending on the difference in coefficient of thermal expansion of the plurality of layers, it bends and deforms in the Z direction, thereby obtaining displacement in the Z direction.

  More specifically, the cantilever has a configuration in which gold is coated on silicon nitride. The gold coating is irradiated with light. Gold absorbs light in the visible light region and has a high coefficient of thermal expansion. Therefore, the displacement of the cantilever can be suitably obtained by using the gold coating.

  Moreover, in order to reflect the light of an optical lever type sensor, the cantilever has been conventionally coated with gold. The present invention is also advantageous in that this gold coating can be utilized.

  In addition, the present embodiment includes a thermal response compensation unit. More specifically, phase compensation is performed by paying attention to the fact that the thermal response shows the behavior of a low-pass filter. The thermal response compensator has a configuration equivalent to the inverse transfer function of the heat transfer function of the cantilever, whereby phase compensation can be performed. Thermal response compensation can apparently eliminate (or significantly reduce) the slow process of heat conduction.

  Therefore, an element that links light irradiation as input and displacement as output is close to the mechanical characteristics of the cantilever itself. That is, the deformation response speed associated with thermal expansion can be made substantially the same as the deformation response speed according to the original mechanical characteristics of the lever structure. As a result, the response speed of deformation due to thermal expansion is increased, and high-speed scanning and high-speed imaging thereby are possible.

"Lever vibration control"
Next, FIG. 12 shows another embodiment. In the above-described embodiment, Z scanning is performed by light irradiation. In this embodiment, in addition to Z scanning, the cantilever is excited by light irradiation. Hereinafter, description of matters similar to those of the above-described embodiment will be omitted.

  In the AFM 101 of FIG. 12, the oscillation circuit of the holder 21 is deleted. Instead, the drive laser control unit 13 includes an oscillation circuit 51. The oscillation circuit 51 is controlled by the computer 15 to generate an excitation signal. This excitation signal is a signal having a vibration waveform of the resonance frequency of the cantilever. This excitation signal is added to the irradiation control signal of the drive laser controller 13. The AC component signal is added to the original DC component irradiation control signal.

  Such an irradiation control signal is supplied to the drive laser unit 11 via the thermal response compensation circuit 35. Therefore, the laser light intensity is modulated at the same frequency as the resonance frequency of the cantilever 5. At the same time, the DC component of the laser light intensity becomes a value corresponding to the reference displacement of the cantilever 5 and is adjusted by feedback control. The displacement of the cantilever 5 also changes according to the laser beam intensity. In this way, the average displacement (center of displacement of the vibrating cantilever) can be feedback controlled while the cantilever 5 is vibrated at the resonance frequency by light irradiation. That is, excitation of the cantilever 5 and Z scanning can be performed by one laser unit.

  The AFM according to the embodiment of FIG. 12 has been described above. In the present embodiment, the oscillation circuit 51 functions as the oscillation control unit of the present invention. According to the present embodiment, both cantilever excitation and Z scanning are performed by light irradiation. Thereby, high-speed scanning becomes possible. In addition, mechanical excitation of the cantilever is not necessary, and the structure can be simplified.

"Q value control"
FIG. 13 shows still another embodiment. In FIG. 13, a configuration for Q value control is further added. Hereinafter, description of matters similar to those of the above-described embodiment will be omitted.

  The Q value (quality factor) is a dimensionless number that represents the state of vibration, and is a parameter that represents the sharpness of the resonance spectrum (relationship between vibration frequency and displacement). The Q value is expressed as ω0 / (ω2−ω1). ω 0 is the resonance frequency at the resonance peak. ω1 and ω2 are frequencies at which the vibration energy becomes half value on the left and right sides of the resonance peak.

  Increasing the response speed of the cantilever contributes to increasing the scanning speed of the AFM. The response speed of all mechanical devices including the cantilever is described by the equation πf / Q using the resonance frequency (f) of the device resonance and the Q value. It can be seen that increasing the resonance frequency or decreasing the Q value is effective for increasing the response speed. However, the resonance frequency and the Q value are values determined by the shape of the cantilever and the measurement environment. It is not easy to manufacture a cantilever whose resonance frequency and Q value are greatly changed.

ｍは質量であり、γは粘性係数であり、ｋはバネ定数であり、Ｆ（ｔ）は外力を表す。一般解は、下記の通りである。

Among these characteristics, for the Q value, a method for effectively controlling the Q value of the resonator is known. The equation of motion of the resonator is written as follows:

m is mass, γ is a viscosity coefficient, k is a spring constant, and F (t) represents an external force. The general solution is as follows.

Here, when the differential value of the general solution signal (displacement signal) is multiplied by a certain amplification factor (α) and added to (or subtracted from) the external force, the equation of motion is written as follows.

This equation is further modified as follows. “Eff” means an execution value.

  From the above calculation, it can be seen that the effective value of the Q value can be changed. It is assumed that such Q value control is applied to the cantilever. In this case, the Q value control is a control in which the cantilever is vibrated by the energy of the driving body, the displacement of the cantilever is detected, and an external force corresponding to the displacement is applied.

  However, in the conventional AFM, the vibrator is composed of a piezoelectric element. In the driving method using the piezoelectric element, the resonator other than the cantilever also vibrates. Therefore, the calculation of the Q value control cannot be performed, and the Q value control is difficult (the difficulty of the Q value control of the piezo element can be understood from the peak forest in the vibration characteristic of the piezo element shown in FIG. ).

  Therefore, in order to perform the Q value control calculation, it is desirable to directly drive only the cantilever with a configuration different from that of the conventional piezoelectric element. If direct drive is possible, it can be said that Q value control becomes easy. Such a requirement is satisfied in the AFM of the present embodiment. That is, the AFM of this embodiment directly drives the cantilever with the drive laser light. In addition, since a thermal response compensation circuit is inserted, a delay in thermal response is compensated. Since the slow process of heat conduction is apparently gone, the original mechanical properties of the cantilever remain. That is, it can be considered that an external force is directly applied to the cantilever (without passing through heat conduction). Utilizing this fact, the AFM of the present embodiment is configured to perform Q value control as described below.

  Referring to FIG. 13, in the AFM 201 of the present embodiment, the drive laser control unit 13 is provided with a Q value control circuit 61. The Q value control circuit 61 receives a displacement signal of the cantilever 5 from the sensor 9. The Q value control circuit 61 processes the displacement signal of the cantilever 5 to generate a Q value control signal. The Q value control signal is added to the irradiation control signal and input to the thermal response compensation circuit 35.

  FIG. 14 shows the configuration of the Q value control circuit 61. The Q value control circuit 61 includes a differentiation circuit (Differential Amplifier) 63 and a gain circuit (Gain Amplifier) 65. The displacement of the cantilever 5 is differentiated by the differentiation circuit 63. Then, a gain is given to the differential value by the gain circuit 65. The signal thus generated is added to the irradiation control signal by the adder 67 as a Q value control signal. Therefore, the Q value control is reflected in the laser beam intensity of the drive laser unit 11, and the Q value control is also reflected in the force generated by the thermal expansion of the cantilever 5. In this way, Q value control of the cantilever 5 is realized.

  As shown in FIG. 13, the Q value control circuit 61 is controlled by the computer 15. The computer 15 controls the coefficient (gain) value in the Q value control circuit 61.

  Next, the effect of Q value control in the cantilever will be described. In the configuration of FIG. 14, if the gain is positive, the effective value of the viscosity coefficient (γ) decreases and the Q value increases as shown in the explanation of the principle of Q value control. Conversely, if the gain is negative, the Q value becomes small.

  When the Q value is reduced (lower), the response speed is improved and the AFM can be increased. Since the control becomes faster, there is an advantage that the damage given to the sample is reduced. However, the sensitivity to the cantilever probe-sample interaction is reduced.

  On the other hand, when the Q value is increased (increased), the response speed decreases. However, the sensitivity to the probe-sample interaction is increased, and the sensitivity of detection is improved. Therefore, the spatial resolution of AFM can be increased.

  15 to 18 show simulation results showing the effect of Q value control. 15 and 16 show simulation results of frequency characteristics. FIG. 15 is a graph of the displacement of the output amplitude with respect to the input amplitude, and FIG. 16 is a graph of the phase. When the Q value control is not performed, the Q value is about 2, and the resonance frequency is 804 kHz. 15 and 16 show three frequency characteristics with a Q value of 0.5, 2, and 10. FIG. As shown in the figure, when the Q value = 10, the amplitude at the resonance frequency increases and the phase changes sharply. From these, it can be seen that increasing the Q value increases the sensitivity.

  17 and 18 show transient response characteristics. FIG. 17 shows an input signal (upper stage), characteristics when the Q value is increased (middle stage, Q value = 10), and characteristics when the Q value control is not performed (lower stage, Q value = 2). FIG. 18 shows an input signal (upper stage), characteristics when the Q value is reduced (middle stage, Q value = 0.5), and characteristics when the Q value control is not performed (lower stage, Q value = 2). . A signal having a frequency of 804 kHz has been input from the point of 20 μs. The signal input is stopped at 70 μs. As shown in the figure, as the Q value increases, the response becomes slower. On the other hand, when the Q value is small, the response is fast.

  The embodiment for performing the Q value control has been described above. In the present embodiment, the Q value control circuit 61 corresponds to the Q value control unit of the present invention.

  According to the present embodiment, since the Q value control unit is provided, Q value control can be performed using light irradiation, and the response speed and sensitivity can be adjusted.

  In the above configuration, Q value control is suitably combined with thermal response compensation. Since the slow process of heat conduction can be apparently eliminated (or significantly shortened) by thermal response compensation, the original mechanical properties of the cantilever itself remain. As a result, light irradiation can produce the same effect as directly exerting a force on the cantilever. Q value control similar to the case of directly applying an external force is performed on the intensity of light irradiation, thereby realizing Q value control. Therefore, Q value control can be easily performed.

  Further, according to the present embodiment, the response speed can be increased and the scanning speed can be improved by decreasing the Q value. Further, the Q value may be increased, the displacement sensitivity can be increased, and the spatial resolution can be improved.

"Other variations"
In addition, the above-described embodiment can be modified by those skilled in the art. For example, in this embodiment, a gold coat is applied to the light irradiation surface of the cantilever. However, materials other than gold may be applied. It is preferable to set the wavelength of the driving laser light according to the material of the light irradiation surface. That is, based on the relationship between the wavelength of the laser beam and the light absorption capability, the wavelength is set so that the light absorption capability is increased. The material of the light irradiation surface is preferably set in consideration of the reflectance of the sensor laser light.

  Regarding the phase compensation in the present embodiment, the configuration of the thermal response compensation circuit may be changed according to the specifications of the cantilever or the like. In the present embodiment, the heat transfer function of the cantilever is expressed by two low-pass filters, and the thermal response compensation circuit is configured to realize an inverse transfer function of the heat transfer function. However, the heat transfer function may vary depending on the configuration of the cantilever or the like. For example, the heat transfer function can vary depending on the shape of the cantilever. Therefore, the configuration of the thermal response compensation circuit may be changed according to the heat transfer function. It is preferable to configure the thermal response compensation circuit so as to realize an inverse transfer function of an actual heat transfer function.

  For example, the heat transfer function may be represented by one low pass filter. The inverse transfer function (1 + derivative) of this heat transfer function may be realized by the thermal response compensation circuit.

  In the above-described embodiment, light irradiation is used for Z scanning, or light irradiation is used for Z scanning and excitation. The present invention also includes a configuration in which light irradiation is used only for excitation. Also in this case, Q value control is suitably performed.

  In the above-described embodiment, the present invention is applied to the AC mode AFM. However, the present invention is not limited to this. The present invention may be applied to a contact mode AFM. In this case, the cantilever does not have to vibrate. And Z scanning is realized by light irradiation.

  In the present embodiment, a piezoelectric element for Z driving (scanning) is not provided. However, a piezoelectric element for driving Z may be additionally provided. In this regard, in the present embodiment, a conventional AFM XYZ scanner may be provided as the XY scanner. In this case, a scanning function only in the XY directions may be used. Alternatively, the scanning function in the Z direction may be used in combination with laser light irradiation. For example, a piezoelectric element may be used to move the cantilever 5 to an appropriate position in the initial stage of operation.

  Furthermore, the present invention can be applied to other than AFM. Another aspect of the present invention is a cantilever driving apparatus and driving method. This aspect may be applied to apparatuses other than the scanning probe microscope.

  Although the presently preferred embodiments of the present invention have been described above, it will be understood that various modifications can be made to the present embodiments and are within the true spirit and scope of the present invention. It is intended that the appended claims include all such variations.

  The scanning probe microscope according to the present invention can increase the scanning speed.

Claims (28)

Cantilevers,
A sensor for detecting the displacement of the cantilever;
A light irradiator that irradiates the cantilever with light to cause thermal expansion deformation;
A light irradiation control unit that changes the displacement of the cantilever by controlling the light intensity of the light irradiation unit;
Wherein the light emission control unit may have line feedback control of the cantilever by controlling the intensity of light of the light irradiation unit based on the detected displacement by said sensor,
The light irradiation control unit includes a thermal response compensation unit that compensates for a delay in the thermal response of the cantilever with respect to light irradiation from the light irradiation unit,
The scanning probe microscope according to claim 1, wherein the thermal response compensator has a configuration equivalent to an inverse transfer function of a heat transfer function of the cantilever .   2. The scanning type according to claim 1, wherein the cantilever has a plurality of layers having different thermal expansion coefficients, and is bent and deformed in a Z direction according to a difference in thermal expansion coefficients of the plurality of layers. Probe microscope.   3. The scanning probe microscope according to claim 2, wherein the cantilever has a configuration in which gold is coated on silicon nitride, and the gold coating is irradiated with light.   The scanning probe microscope according to claim 1, wherein the light irradiation control unit further vibrates the cantilever by controlling light intensity of the light irradiation unit. The scanning probe microscope according to claim 4 , wherein the light irradiation control unit further includes a Q value control unit that performs Q value control based on a displacement detected by the sensor. The scanning probe microscope according to claim 5 , wherein the Q value control unit has a configuration for obtaining a value obtained by adding a gain to the differential of the displacement detected by the sensor. The scanning probe microscope according to claim 5 , wherein the Q value control unit decreases the Q value. The scanning probe microscope according to claim 5 , wherein the Q value control unit increases the Q value. Cantilevers,
A light irradiator that irradiates the cantilever with light to cause thermal expansion deformation;
A light irradiation control unit that changes the displacement of the cantilever by controlling the light intensity of the light irradiation unit;
The light irradiation control unit includes an oscillation control unit that vibrates the cantilever by controlling light intensity of the light irradiation unit, and compensates for a delay in thermal response of the cantilever with respect to light irradiation from the light irradiation unit. and a thermal response compensating section that,
The scanning probe microscope according to claim 1, wherein the thermal response compensator has a configuration equivalent to an inverse transfer function of a heat transfer function of the cantilever . A sensor for detecting the displacement of the cantilever;
The scanning probe microscope according to claim 9 , wherein the light irradiation control unit includes a Q value control unit that performs Q value control based on a displacement detected by the sensor. The scanning probe microscope according to claim 10 , wherein the Q value control unit has a configuration for obtaining a value obtained by adding a gain to a differential of the displacement detected by the sensor. The scanning probe microscope according to claim 11 , wherein the Q value control unit decreases the Q value. The scanning probe microscope according to claim 11 , wherein the Q value control unit increases the Q value. Move the cantilever and sample relative to each other in the XY direction,
Irradiating light to the cantilever to cause thermal expansion deformation,
Detecting the displacement of the cantilever,
There line feedback control of the cantilever by controlling the intensity of the light based on the displacement of the detected the cantilever,
A scanning probe microscope that compensates for a delay in the thermal response of the cantilever to the light irradiation by performing phase compensation using an inverse transfer function of the heat transfer function of the cantilever in the light intensity control. Observation method. 15. The observation method using a scanning probe microscope according to claim 14 , further comprising vibrating the cantilever by controlling the intensity of the light. The observation method using a scanning probe microscope according to claim 15 , further comprising performing Q value control based on the detected displacement of the cantilever by controlling the intensity of the light. Move the cantilever and sample relative to each other in the XY direction,
Irradiating light to the cantilever to cause thermal expansion deformation,
The cantilever is vibrated by the intensity control of the light , and the phase compensation is performed using the inverse transfer function of the heat transfer function of the cantilever in the intensity control of the light, so that the thermal response of the cantilever to the light irradiation is An observation method using a scanning probe microscope, characterized by compensating for the delay. Detecting the displacement of the cantilever,
18. The observation method using a scanning probe microscope according to claim 17 , wherein Q value control based on the detected displacement of the cantilever is performed by controlling the intensity of the light. A sensor for detecting the displacement of the cantilever;
A light irradiator that irradiates the cantilever with light to cause thermal expansion deformation;
A light irradiation control unit that changes the displacement of the cantilever by controlling the light intensity of the light irradiation unit;
Wherein the light emission control unit may have line feedback control of the cantilever by controlling the intensity of light of the light irradiation unit based on the detected displacement by said sensor,
The light irradiation control unit includes a thermal response compensation unit that compensates for a delay in the thermal response of the cantilever with respect to light irradiation from the light irradiation unit,
The thermal response compensator has a configuration equivalent to an inverse transfer function of a heat transfer function of the cantilever. The cantilever driving apparatus according to claim 19 , wherein the light irradiation control unit further vibrates the cantilever by controlling light intensity of the light irradiation unit. 21. The cantilever drive device according to claim 20 , wherein the light irradiation control unit further includes a Q value control unit that performs Q value control based on a displacement detected by the sensor. A light irradiator that irradiates the cantilever with light and causes thermal expansion deformation;
A light irradiation control unit that changes the displacement of the cantilever by controlling the light intensity of the light irradiation unit;
The light irradiation control unit compensates for a delay in thermal response of the cantilever with respect to the light irradiation from the light irradiation unit and an oscillation control unit that vibrates the cantilever by controlling the light intensity of the light irradiation unit and a thermal response compensating section that,
The thermal response compensator has a configuration equivalent to an inverse transfer function of a heat transfer function of the cantilever. The cantilever driving apparatus according to claim 22 , wherein the light irradiation control unit further includes a Q value control unit that performs Q value control based on a displacement of the cantilever detected by a sensor. Move the cantilever and sample relative to each other in the XY direction,
Irradiating light to the cantilever to cause thermal expansion deformation,
Detecting the displacement of the cantilever,
There line feedback control of the cantilever by controlling the intensity of the light based on the displacement of the detected the cantilever,
A cantilever driving method characterized by compensating for a delay in the thermal response of the cantilever to the light irradiation by performing phase compensation using an inverse transfer function of the heat transfer function of the cantilever in the light intensity control . The cantilever driving method according to claim 24 , further comprising vibrating the cantilever by controlling the intensity of the light. 26. The cantilever driving method according to claim 25 , further comprising performing Q value control based on the detected displacement of the cantilever by controlling the intensity of the light. Move the cantilever and sample relative to each other in the XY direction,
Irradiating light to the cantilever to cause thermal expansion deformation,
The cantilever is vibrated by the intensity control of the light , and the phase compensation is performed using the inverse transfer function of the heat transfer function of the cantilever in the intensity control of the light, so that the thermal response of the cantilever to the light irradiation is A cantilever driving method characterized by compensating for the delay. Detecting the displacement of the cantilever,
28. The cantilever driving method according to claim 27 , wherein Q value control based on the detected displacement of the cantilever is performed by controlling the intensity of the light.

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